Recent investigations in our laboratory demonstrate that murine lymphoid cells have the capacity to recognize and respond to antigens of syngeneic tumor cells in an entirely in vitro culture system. Experiments will be performed to determine the role of T lymphocytes, B lymphocytes and macrophages in the initial recognition of tumor specific transplantation antigens (TSTA) in this in vitro system. We will also ascertain the role of these cell types in lysis of tumor cells following in vitro sensitization. In addition, studies will be conducted to determine the relationship between TSTA and normal cell surface antigens which are controlled by the major histocompatibility complex. It is anticipated that these studies will yield new information concerning the mechanisms of host defenses against neoplasia.